1. Field of the Invention
The present invention concerns a procedure and a device for the three-dimensional electroerosive (EDM) or electrochemical (ECM) machining of bent or uneven surfaces of a work piece by means of an electrode which is generally independent of the form of the bent surface.
2. Description of Related Art
Electroerosion devices are known as state of the art in which a slim electrode xe2x80x9cmillsxe2x80x9d a bent surface, whereby the shape of the electrode is not dependent on the bent surface. For this purpose, the work piece is clamped on a work table which, if necessary can be moved in the X, Y or Z direction. To produce the bent surface, the electrode is led in the direction of the work piece, erosion impulses in the form of voltage impulses are placed against a die electrode and the processing gap between electrode and work piece is rinsed. The electrode is led across the work piece surface like a milling machine and pares off the individual sections of the work piece to be eroded layer by layer to form the desired bent surface. This procedure promotes a high degree of wear and tear which has to be compensated for.
Such an electroerosion device is known from U.S. Pat. No. 4,505,721, operating with a hollow electrode through which the rinsing liquid flows during the machining into the machining gap.
From the summary of the Japanese Patent Publication No. JP-10-128 624 another electroerosion device is known in which the electrode for the erosion of a bent surface is tracked in its angle of inclination during the forward feed motion. The electrode is led in such a manner that it is always vertical to the tangential direction of the forward feed direction.
It is basically impossible to prevent wear and tear on the electrodes during the erosion procedure, which has the effect that the shape of the electrode changes during the machining. If the electrode is also set into rotation during the machining, the result is, for example, the wear profile on the electrode as shown in the right lower half of FIG. 1a. 
From EP 555 818 A1 an electroerosion device is known, for example, on which the electrode is set into rotation. It also calculates the wear and tear on the electrode during the machining and from this the correction for the forward motion directions derived, which is shown as an overlay of the forward motion in the X, and Y plane and a tracking motion of the electrode in the Z direction. As is described in this publication, the wear profile changes in dependence of several parameters. However, this also changes the working surface of the electrode in dependence of the wear and tear condition. Thus, with a given generator setting, there is a certain wear and tear (use rate) and a corresponding wear profile.
In order to maintain a machining precision, the wear and tear has to be kept to as constant a level as possible. For this reason, it is only possible to remove layers that are progressively thinner during sequential machining steps in the direction of smoother and smoother surfaces (up to fine finishing).
It is also known, for example, to measure the electrode wear and tear and to compensate by tracking of the electrode according to the measured values. Such an electroerosion device is known, e.g., from DE 30 36 462 A1, in which the electrodexe2x80x94in addition to its motion in the X, Y and Z directionsxe2x80x94is also set to vibrate during the xe2x80x9cmillingxe2x80x9d of the concave surface. The vibrations are then registered by a detection device for the purpose of the determination of the wear and tear of the electrode.
In this device, the minimum radius which can be eroded is dependent on the width of the machining gap and the radius of the electrode. For this reason, only electrodes with a small diameter, thus thin cylindrical electrodes with a small working surface are used. To make it possible to still attain a sufficient rate of erosion, the electrode is set in rotation to support the rinsing. This measure allows increasing the maximum admissible current density and thus also the paring rate.
However, thin electrodes have the disadvantage that they are not rigid enough and bend under the effect of the rinsing and erosion forces which reduces the machining precision. In addition, a possible collision between the thin electrode and the work piece etc. can lead to a plastic reshaping of the electrode.
In general, the known devices have the following disadvantages: the electrode profile or the electrode geometry changes during the processing, in particular, with lateral electrode wear and tear. In addition, the electrodes are not sufficiently rigid against bending. For this reason, the attainable machining precision is severely limited in the known devices.
From DE 33 36 034 C2 an electrical discharge erosion device is known in which a rod-shaped electrode with circular or angular cross section is moved along a pre-programmed forward feed track. The forward feed tracks run layered in parallel planes. During faulty operation the electrode is moved away from the surface to be machined along a retraction path designed for each machining point. Due to the retraction paths, which have been calculated in advance, there are no unnecessary time delays during the retraction motion.
From DE 32 03 605 A1 another electrical discharge erosion device with a rotatable electrode is known. Parallel to its lengthwise axis, the electrode has several differing machining sides (varying profiles) which can be called upon, depending on the rotation position, for the machining of the work piece.
An object of the present invention is to maintain the electrode profile essentially constant during machining.
According to the present invention, when an electrode is led in a forward feed direction tangentially to the surface of the work piece, the lengthwise axis of the electrode is inclined away from a normal to the work piece surface such that the end not facing the work piece is directed away from the forward feed direction. The advantage gained with this, as compared to the state of the art, is that the working surface of the electrode can be twice as large. This means that with the same current density, the current can suitably be doubled and thus the paring rate can be more than doubled. This can be clearly seen, e.g., from the comparison of FIGS. 1a and 1b to FIGS. 1d and 1e. On the electrode shown in FIG. 1a from the state of the art, the working surface is solely located on the bottom surface of the electrodexe2x80x94seen in the forward feed direction of the electrodexe2x80x94in the front section. Based on the wear of the electrode in the front section and simultaneous rotation of that section towards the rear, the rear section of the bottom surface of the electrode no longer participates in the erosion. However, in the invention, due to the incline of the lengthwise axis of the electrode from the normal of the work piece surface, the full bottom surface of the electrode participates in the erosion.
This also considerably improves the rinsing conditions. The maximum pressure drop is no longerxe2x80x94as in the state of the artxe2x80x94at the rear of the section of the wider machining gap, not used for the erosion, but equally distributed over the full active machining gap.
Another decided advantage is that the profile of the electrode only changes very little during its wear. The electrode wears off evenly along its bottom surface which allows simple compensation by the respective pushing up of the electrode. It does not change the profile.
In this manner the decisive advantage of an electroerosion device as compared to a mechanical milling device one can be further exploited, i.e., no relative motion between the work piece and the tool in the form of a rotation or similar is needed. This advantage in particular could not sufficiently be utilized with the known electroerosion devices as they had to be rotated in principle due to the uneven wear of the electrode. Such a rotation is no longer necessary with this invention.
This also opens another great advantage of the invention, i.e., that it is possible to work with prismatic electrodes which may not be rotated during machining. With a suitable prism shape (with sharp outside edges) it is thus possiblexe2x80x94contrary to the state of the artxe2x80x94to advantageously erode a great number of smaller radii, e.g., a great number of narrow corners etc.
In addition, for the erosion of small radii it is an advantage that specially thin electrodes are no longer needed. With the use of prismatic electrodes it is possible to retain a large cross section on them which means that larger working surfaces and an overall greater rigidity of the electrode. Thus, it is possible to again pare off three-dimensional surfaces with greater speed and precision.
The term xe2x80x9cforward feed direction of the electrodexe2x80x9d means that direction in which the electrode holder is moved to advance on a given erosion track. Instead of or in addition to the motion of the electrode holder it is also possible to move the tool table with attached tool accordingly. The term xe2x80x9cwork piece surfacexe2x80x9d refers to the possibly bent, unmachined surface section which is located in the area of the electrode and is supposed to be treated next.
In order to obtain a bent surface according to the process of the invention, at least one other axis to incline the electrode is needed.
In general the shape of the electrode is independent of the shape of the bent surface to be machined. However, it can be an advantage in many cases to adapt the electrode to the shape of a particularly difficult to machine section of the bent surface. This means that the electrode for smaller segments of the bent surface may very well be dependent on its shape.
It is preferred to adjust the ejection angle in dependence of the machining quality, whereby the particularly preferred ejection angle is returned towards zero during the last fine machining. In an advantageous manner it is here possible to increase the paring rate for the coarser machining steps by a corresponding wider angle or incline of the electrode, whereby the inclination angle is reduced in favor of machining precision for the fine machining steps.
It is preferred that material is removed from the depth of the work piece in layers.
In one embodiment of the process it is preferred that the electrode is lifted off the surface of the work piece along the direction of its lengthwise axis and then repositioned on the work piece surface tangentially to the work piece surface. This measure permits a particularly rapid interruption of the process embodiment and the progressive restart of the erosion.
In a preferred manner, the electrode is rotated at intervals or continuously during the machining. It is preferred in the case of electroerosive treatment with a prismatic electrode that the electrode be retracted from the work surface before rotation. The advantage is that in this manner compensation is made even for the last remaining irregular wear and tear of the electrode.
For an advantageous increase in the speed of the rinsing liquid, the electrode in electroerosive or electrochemical machining is set to vibrate.
In a preferred manner, the electrode is centrally rinsed, whereby for rinsing the electrode sucks in the rinsing liquid and the rinsing pressure is controlled. This combination of central rinsing and sucking in of the rinsing liquid has the advantage to attain, on the one hand, a high rinsing liquid speed and, on the other hand, obtain a pressure drop primarily in the erosion gap.
In the case of electroerosive machining the duration of the erosion impulses, particularly during pressure rinsing is preferably selected somewhat lower than the flow time of a discharge that would arise in the electrode center and extinguish itself independently at the electrode edge. In an advantageous manner it would be possible with this measure to select the erosion impulse to be as long as possible, almost to that of direct current erosion.
In a preferred manner the erosion voltage is monitored for change in the electric arc voltage and/or in the overriding maintained wave and on deviation of the set value, the erosion impulse is canceled. A damaging of the electrode can in this manner be advantageously prevented.
In a preferred manner the electrode is coated with a lateral insulation layer. This insulation layer, which preferably is very thinly applied, acts to prevent damage to the lateral wall of the electrode. Such an electrode can also be used independent of the device according to the invention, e.g., in known electroerosion devices, in order to reduce the lateral wear of the electrodes there.